Photoelectrochemical air disinfection

ABSTRACT

A system for disinfecting a fluid containing microorganisms or chemical contaminants includes a plurality of photocatalyst surfaces secured to a solid surface upon which a fluid to be disinfected contacts. A structure for removing a portion of the photogenerated electrons is in electrical contact with the photocatalyst layer, wherein an electron-hole recombination rate involving the photogenerated electrons and holes is reduced, thus increasing the removal rate of microorganisms or chemical contaminants from the fluid. The system can include a source of photons having a wavelength corresponding to a band gap energy of the photocatalyst to illuminate the photocatalyst layer. The invention can be used in air supply registers of a heating, ventilating and air conditioning system, or in air ducts, or used to disinfect wall coverings, floor coverings, envelopes, packages, and clothing articles.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

The invention relates to systems and methods for disinfecting fluidsusing photoelectrochemical processes enhanced by electron shunting.

BACKGROUND

Several microbiological particle control systems exist. Mechanical andelectrostatic filters can be used to reduce indoor concentrations ofrespirable particles, such as in a forced air heating/cooling system ofa building. Microbiological filters have been used for disinfection ofair and other gases because of their low cost and ease of handling.These filters can be constructed to remove not only microorganisms, butalso submicron particles as well. For efficient and economic operationof these filters, the aerosol content of the air to be filtered must below, where the term “aerosols” generally refers to microorganisms,particles, and droplets of liquid dispersed in air. A disadvantage ofsuch filters is that they do not permanently remove the contaminants,but just transfer them to the filter medium. Clogging of the filtermedium can result which can cause high pressure drops. in addition,microorganisms trapped on the filter media continue to multiply makingthe filter media a breeding ground and thus hazardous.

Another method of microbiological particle removal is UV disinfection.UV disinfection has been widely used to destroy biological contaminantsand toxic chemicals. Such UV treatment has worked well for disinfection,but the indoor environment may also be contaminated with low level toxicchemicals such as formaldehyde, styrene, and toluene. Ultraviolet energyalone has proven generally ineffective in degrading these chemicals. Forinstance, U.S. Pat. No. 5,045,288 to Raupp and Dibble, and U.S. Pat.Nos. 4,892,712; 4,966,759; and 5,032,241 to Robertson and Hendersondisclose use of UV to treat fluids and gases that contain pollutants.

Another alternative that has gained increasing attention isphotocatalytic oxidation (PCO), which involves the use of aphotocatalyst such as TiO₂ for the destruction of hydrocarbons andmicroorganisms in fluids. Titanium dioxide (TiO₂) is a semiconductorphotocatalyst with a room temperature band gap energy of about 3.2 eV.When this material is irradiated with photons having wavelengths lessthan about 385 nm (UV), the band gap energy is exceeded and electronsare generated through promotion from the valence band to the conductionband which results in the generation of holes (h+). The resulting highlyreactive electron-hole pairs have lifetimes in the space-charge regionof the photocatalyst that enables participation in chemical reactions,provided recombination events do not occur first. The most widelypostulated chemical reactions are shown below:OH⁻ +h+.OH (hydroxyl radical)O₂ +e−→O ₂ ⁻ (super-oxide ion)

Hydroxyl radicals and super-oxide ions are highly reactive species thatcan readily oxidize volatile organic compounds (VOCs) adsorbed oncatalyst surfaces. They can also kill and oxidize adsorbed bioaerosols.The process is a form of heterogeneous photocatalysis, or morespecifically PCO.

Several attributes of PCO make it a strong candidate for indoor airquality applications. Pollutants, particularly VOCs, are preferentiallyadsorbed on photocatalytic surfaces and oxidized primarily to carbondioxide (CO₂). Thus, rather than simply changing the phase andconcentrating the contaminant, the absolute toxicity of the treated airstream is reduced, allowing the photocatalytic reactor to operate as aself-cleaning filter.

Photocatalytic reactors may be integrated into new and existing heating,ventilation, and air conditioning (HVAC) systems due to their modulardesign, room temperature operation, and generally negligible pressuredrop. PCO reactors also feature low power consumption, potentially longservice life, and low maintenance requirements. These attributescontribute to the potential of PCO technology to be an effective processfor removing and destroying low level pollutants in indoor air,including bacteria, viruses and fungi.

For example, U.S. Pat. No. 5,933,702 to the same inventor as the presentapplication discloses a method for disinfecting an air stream containingmicroorganisms. The method includes the steps of providing an air streamcontaining microorganisms having a relative humidity greater than about40% and contacting the air stream with a photocatalyst having apredetermined band gap energy in the presence of a source of photonshaving a wavelength corresponding to the band gap energy of thephotocatalyst. At least a portion of the microorganisms in the airstream are destroyed by photocatalytic oxidation. Improved devicesembodying the method of the invention are disclosed, such as stand-alonedevices and devices incorporated into the HVAC systems of buildings,including the air supply registers. Photocatalyst-coated filter mediacapable of trapping bioaerosols are also disclosed. U.S. Pat. No.5,933,702 is hereby incorporated by reference into the currentapplication in its entirety.

However, even the improved methods and devices disclosed in U.S. Pat.No. 5,933,702 have efficiencies which are limited by a generally slowphotocatalytic oxidation process. The slow photocatalytic oxidationprocess results mainly because a large percentage of photo-generatedelectrons recombine with photo-generated holes before the holes have achance to participate in a photocatalytic reaction.

SUMMARY OF THE INVENTION

A system for disinfecting a fluid containing microorganisms or chemicalcontaminants includes a plurality of photocatalyst surfaces secured to asolid surface upon which a fluid to be disinfected contacts. The systemcan be disposed in an air supply register of a heating, ventilating andair conditioning system or in an air duct. A structure for removing aportion of the photogenerated electrons is in electrical contact withthe photocatalyst layer and provides an electrically conductive path forphoto generated electrons to move away from photo generated holes. As aresult, the electron-hole recombination rate involving thephotogenerated electrons and holes is reduced, thus increasing thequantum efficiency of the photocatalytic oxidation process and thedestruction rate of microorganisms or chemical contaminants present inthe fluid.

The system can include a source of photons having a wavelengthcorresponding to a band gap energy of the photocatalyst to illuminatethe photocatalyst layer. The photocatalyst layer can include a pluralityof metal ions, such as Ag+. The fluid can be a liquid or an air stream.

The structure for removing a portion of the photogenerated electrons cancomprise an electron conducting path such as a metallic grid in contactwith the photocatalyst layer, the metallic grid including a plurality ofmetallic strips. Gaps between the strips allow photons to reach thephotocatalyst. The plurality of metal strips can be connected to anexternal ground. Alternatively, a source of electrical bias connected tothe metal grid, the bias increasing the rate of removal of thephotogenerated electrons.

The structure for removing a portion of the photogenerated electrons cancomprise at least one material generally toxic to microorganisms, suchas silver.

A method for disinfecting fluids includes the steps of providing aphotocatalyst layer comprising a plurality of photocatalyst particles,illuminating the photocatalyst layer with a source of photons having awavelength corresponding to a band gap energy of the photocatalyst,wherein photogenerated electrons and holes are produced. A portion ofthe photogenerated electrons are removed, wherein a lower concentrationof photogenerated electrons remain for electron-hole recombinationreactions increasing removal efficiency of microorganisms or chemicalcontaminants from the fluid stream. The fluid stream is contacted withthe photocatalyst layer, wherein at least a portion of themicroorganisms or chemical contaminants in the fluid stream arephotocatalytically oxidized.

A system for disinfecting air comprises a duct through which air ismoved, a blower connected to the duct to move air through the duct, aphotocatalyst layer including a plurality of photocatalyst particleshaving a predetermined band gap energy secured to a solid surface in theduct, structure for directing the photogenerated electrons away fromphotogenerated holes disposed in contact with the photocatalyst layer,and a source of photons for illuminating the photocatalyst with photonshaving a wavelength corresponding to the band gap energy of thephotocatalyst. The solid surface can comprise at least one interior wallof the duct, the photocatalyst layer and the structure for directingbeing coated onto the wall.

A coating for disinfecting surfaces comprises a photocatalyst layerincluding a plurality of photocatalyst particles for photogeneratingelectrons and holes, the photocatalyst particles secured to a solidsurface, and structure for removing a portion of the photogeneratedelectrons, wherein an electron-hole recombination rate involving thephotogenerated electrons and holes is reduced increasing a removal rateof microorganisms or chemical contaminants from the surface. The solidsurface can comprise wall coverings, floor coverings, envelopes,packages or clothing articles.

A method for disinfecting surfaces comprises the steps of providing asurface including a photocatalyst layer comprising a plurality ofphotocatalyst particles secured to the surface for photogeneratingelectrons and holes, contacting an electrically conducting structure tothe photocatalyst layer, and illuminating the photocatalyst layer withUV light, wherein a portion of electrons generated by the photocatalystparticles are shunted by the electrically conducting structure. Theproviding step can comprise applying the photocatalyst layer to wallcoverings, floor coverings, envelopes, packages, or clothing articles.The contacting and illuminating steps can be automated.

BRIEF DESCRIPTION OF THE DRAWINGS

A fuller understanding of the present invention and the features andbenefits thereof will be accomplished upon review of the followingdetailed description together with the accompanying drawings, in which:

FIG. 1 is a diagrammatic-schematic of a typical heating/cooling ductsystem for a building with a reactor system having this inventionincorporated therein, according to an embodiment of the invention.

FIG. 2 is an exploded diagrammatic view illustrating components of afirst embodiment of the reactor unit which includes an electricalconnection between a metal grid which is electrically coupled to thefront portion of photocatalyst particles and the back of thephotocatalyst particles, according to an embodiment of the invention.

FIG. 3 is an exploded diagrammatic view illustrating components of asecond embodiment of the reactor unit which includes an electricalconnection and an applied DC bias between the front and back portions ofphotocatalyst particles, according to an embodiment of the invention.

FIG. 4 is a longitudinal cross-sectional view of a stand-aloneembodiment of the invention.

FIG. 5 illustrates a water purification system based on an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

A photocatalytic reactor-based system and related method fordisinfecting fluids containing microorganisms or chemical contaminantsincreases the speed and efficacy of contaminant destructionsignificantly compared to previous systems by significantly reducing therate of hole-electron recombination reactions. In contrast to previousadvanced photocatalytic-based systems such as U.S. Pat. No. 5,933,702which requires minimum relative humidity levels of 40% or more toachieve reasonable rates for the photocatalysis process, the presentinvention can operate efficiently at much lower relative humidity levelsbecause of the reduced recombination. Effectively a larger number ofactive reactive sites per unit photocatalyst particle are provided bythe invention. Accordingly, the invention can be used even when relativehumidity is less than about 30%, while still providing substantiallycomplete disinfection of air. Although, the invention is generallyapplied to air or other gases, the invention can be adapted for use indestroying contaminants present in liquid media, such as water.

FIG. 1 is a diagrammatic-schematic of a typical heating/cooling ductsystem 100 for a building with a reactor system having this inventionincorporated therein, according to an embodiment of the invention. Inmost buildings, a blower/fan causes the air from the various zones of anair conditioned space to be drawn into a duct system via inlet openingsand particle/aerosol filters 12. The air then can pass over the heatingcoil of the furnace or the heating/cooling coil of an airconditioner/heat pump of the air conditioning unit 14.

The fan 65 of the air handling unit 14 forces the air passing over coils56 and 58 into a duct system 18. Detector 52 detects the humidity levelin the air. Coils are used to condense water vapor, if the relativehumidity in the air is above a predetermined level, such as 70%. Waterspray or atomizer 54 adds moisture to the air stream if the humiditylevel measured by detector 52 is below a predetermined level, such as30%.

In FIG. 1 there is a master reactor 21 along the duct 18. In manyinstallations this will be sufficient. However, in the embodiment shownin FIG. 1, there is also shown a series of reactor units 22 disposed inbranch lines of duct system 18. Reactor units 21 and 22 include aplurality of photocatalyst particles secured to a solid surface and atleast one UV light source and structure for removing photogeneratedelectrons, exemplary embodiments being shown in FIGS. 2 and 3.

In FIG. 1 a conventional flow or speed detector 44, such as a MamacAnubar flow detector can be located in the main duct system 18. Speeddetectors are oftentimes placed within each reactor 22 and it is thattype of reactor which is described with respect to the stand-alone unitsdescribed hereinafter.

The faster the air speed, the less time air will be retained over thecatalytic surfaces. As speed or volumetric displacement is lowered,retention time increases. It is usually desirable to maintain airmovement throughout the building at all times. Here, the air speed canbe adjusted for a maximum destruction of the deleterious matter bycontrolling the retention time over the photocatalytic surface.

This retention time will vary depending on the air flow rate, the sizeof the ducts, the area of the catalytic surfaces and other physicalcharacteristics. The air speed or volume flow rate is preferably enteredinto a microprocessor 62 from the detector 44. The microprocessor 62 inturn will control the speed of fan motor 64 and thus the airdisplacement of fan 65. The blower speed can be adjusted to provide therequired residence time.

FIG. 2 diagrammatically illustrates components of a first exemplaryreactor unit 200 which can be used as reactors 21 and 22 shown inFIG. 1. These components include a catalytic filter 205 and a bank oflamps 210. At least a portion of the photons emitted by lamps 210 havean energy at least equal to the band gap energy of the photcatalyst. Intypical applications, lamps 210 preferably deliver low energy photons ofthe UV-A and lower energy portion of the UV-B spectrum. A UV wavelengthbetween about 300 and about 400 nm is generally preferred. For example,low pressure mercury lamps (Southern New England RPR-3500A) with aninput energy of 14 W can be used. Each lamp emits approximately 1.5 W ofUV-radiation, predominately at about 350 nm.

Light from UV lamps 210 is directed towards catalytic filter 205.Catalytic filter 205 includes alternating photocatalyst coatedcorrugated substrate elements 211 and photocatalyst coated planarsubstrate elements 212. A photocatalyst layer 208 comprising a pluralityof photocatalyst particles, such as TiO₂ particles, are secured to atleast one side of substrate elements 211 and 212. Although photocatalystlayers 208 are shown disposed on only one side of filter substrateelements 211 and 212, photocatalyst layers may be disposed on both sidesof corrugated substrate elements 211 and 212. Gaps (not shown) areprovided between photocatalyst layers 208 in photocatalytic filter 205to provide sufficient air space to permit air passage therethrough witha minimum of pressure drop.

In one embodiment, substrates 211 and 212 can be a fibrous media, suchas a fibrous woven or non-woven material, analogous to common airconditioning filters. Substrate 211 and 212 can be HEPA or HEPA-like. Atleast some of the fibers are preferably electrically conducting fibers,such as metal fibers, electrically conducting polymer fibers,electrically conducting carbon fibers or carbon nanotubes. Some examplesof non-electrically conducting fibers and materials for the substrate211 and 212 which are preferably mixed with electrically conductingfibers include natural fibers such as cotton and wool, man-made andsynthetic fibers such as rayon, polyester, polypropylene andpolytetrafluoroethylene, and other materials such as flame resistantfibrous materials and carbons and all other functional fibrousmaterials.

Catalytic filter 205 includes structure for significantly reducinghole-electron recombination in the form of an electrically conductiveshunting structure to remove photogenerated electrons before they canrecombine with photogenerated holes. Each photocatalyst layer 208 can beprovided its own shunting structure. FIG. 2 shows electricallyconductive (e.g. metallic) grids 230 disposed on photocatalyst layers208, grids 230 comprising a plurality of spaced apart electricallyconductive strips 232. Spacing between conductive strips 232 is providedto allow UV light to reach the photocatalyst particles comprisingphotocatalyst layer 208.

However, if optically transparent electrically conductive materials areused, grid 230 can be replaced by a continuous sheet (not shown). Forexample, fluorine doped SnO₂ is known to be substantially opticallytransparent in the UV range from the violet edge of the visible spectrum(about 400 nm) to about 260 nm.

The strips 232 comprising grid 230 are preferably thin and narrowmetallic strips. Grid 230 is connected to an electrically conductivepathway to facilitate removal of electrons generated by thephotocatalyst particles comprising photocatalyst layer 208 by allowingphotogenerated electrons which reach grid 230 to be shunted away. Shuntwire 240 electrically connects grid 230 to the back of substrates 211and 212. A single shunt wire 240 can make electrical connection to theplurality of substrates 211 and 212, or each substrate 211 and 212 canbe provided a separate shunt wire 240.

Optionally, in applications such as industrial applications where theelectron flow generated is comparatively large, one or more low valueresistors (not shown) can be placed in the pathway of shuntwire 240 topermit utilization of the electron flow in shunt wire 240. For example,a resistor having a value of several hundred milliohms to several ohmscan be used.

If filter substrates 211 and 212 are not electrically conductive, thesubstrates 211 and 212 can be coated with an electrically conductivelayer (not shown), such as aluminum. In this embodiment, shunt wire 240would connect grid 230 to the electrically conductive coating layer.Alternate shunting arrangements will be apparent to those havingordinary skill in the art. For example, metallic grid 230 can beconnected to a common external ground via a wire connector (not shown).Thus, in any of the above described embodiments, reactor unit 200directs a substantial portion of the photogenerated electrons byphotocatalyst layer 208 away from photocatalytically generated holesavoiding electron-hole recombination reactions, thus increasing thequantum efficiency of reactor unit 200.

Essentially any material capable of catalyzing photocatalytic oxidationwhen illuminated with a source of photons in the presence of air havinga relative humidity greater than about 15 to 20% is suitable for use asa photocatalyst in the present invention. Such materials are readilyidentified by those of ordinary skill in the art without undueexperimentation. Examples of suitable photocatalysts are semiconductormaterials such as ZnO₂, TiO₂, and the like.

A thick slurry of TiO₂ (or other photocatalyst) can be made by mixingTiO₂ (or other photocatalyst) powder and deionized water. The TiO₂solution is then mixed well by placing a flask with the thick TiO₂slurry on a magnetic stirrer. Using a foam brush, the TiO₂ can be coatedon one or both sides of solid surfaces 225. The solid surfaces 225should be dried, such as for six hours, then another coat of TiO₂ can beapplied. A heat gun can be used to accelerate the drying of thesurfaces. After the solid surfaces are dry and the TiO₂ is sufficientlyaffixed, the catalytic filter 205 is ready for use, such as in arecirculating duct. However, heat treatment of the coated substrate atan appropriate temperature may be necessary to secure the coating to thesubstrate.

The present invention also contemplates providing a photocatalyst-coatedsurface in a duct section by painting the interior duct walls with thephotocatalyst coating and laying a metallic grid 230 or equivalentstructure thereon. UV lamps are then installed in the duct to illuminatethe photocatalyst-coated interior duct walls.

The current flowing along the metallic grid 230 can be utilized for avariety of useful purposes. For example, electrons collected can be usedin reduction reactions, such as to remove metal oxides from water.Electrons collected can also be used to generate light or collected in asuitable energy storage unit for later use.

The metallic grid 230 preferably includes materials that are toxic tomicro-organisms, such as the element silver. Silver can be provided inelectrically conducting compound alloys, and include Ag containingcompounds such as silver nitrate. Other materials toxic to themicroorganisms of interest can also generally be used with theinvention. Thus, microorganisms entering reactor unit 200 will die oncontact with metallic grid 230 and will then be oxidized by enhancedphotocatalysis provided by reactor 200. Metal ions may also be dispersedin the catalyst layer 208 to accelerate the kill rate of micro-organismsand increase the rate of photocatalysis further.

The metallic grid 230 may be formed by a variety of known methods. Forexample, screen printing or vapor deposition can be used. In addition,metal grid 230 may be formed from precipitation of metal from suitablemetal compounds by action of light or some other reaction.

FIG. 3 shows a reactor 300 according to an alternate embodiment of theinvention. Reactor 300 include a power supply 305 for providing a DCbias across photocatalyst layers 208. This arrangement can furtherimprove the efficiency of the disinfection process by drivingphotogenerated holes and electrons in different directions. Althoughgenerally not preferred, an AC bias may also be used.

The applied bias is preferably sufficient to keep the electronsseparated from the holes in the photocatalyst by directing them indifferent directions. It may also be possible for the anode bias to besufficient in the absence of a photochemical reaction to causeelectrochemical oxidations. However, since the photocatalyst material isa semiconductor with high energy band gap, such as 3.2 eV for TiO₂, avery large bias would generally be needed to provide sites forelectrochemical oxidation in the absence of light of appropriatewavelength. Magnetic separation of electrons and holes through providinga magnetic field may also be used either alone or in conjunction withstructures described above (not shown).

The essentials of the invention can be utilized independent of a ductsystem. Such a stand-alone unit 400 is shown in FIG. 4. The unit 400includes a housing 420 having an inlet 424 and an outlet 426.Intermediate the inlet and outlet is a chamber 428 that includes acontrol fan 429, a fan motor 464 for driving fan 448, and catalyticfilter 496 upon which UV light is directed from a series of UV lamps498. An air speed detector 499 determines the air flow displacement. Asecond chamber 410 is provided on the housing 420 to enclose amicroprocessor 402. The electric power for the unit 400 is provided tothe system through the control processor 402 via leads 404 and 406. Thestand-alone unit 400 may be operated without the airspeed detector 499,if the unit 400 is designed and constructed with a constant air flowrate fan 448 matched with the cross sectional area of the housing 420 toprovide the desired residence time for the air flow over the catalyst.

A particulate prefilter 485 is provided to maintain the interior of thestand-alone unit free of dirt that might damage catalytic filter 496.Catalytic filter can be a filter based on the inventive arrangementshown in FIG. 2 or FIG. 3. The particulate prefilter can be anycommercially available unit, or a commercially available unit coatedwith photocatalyst particles.

The stand-alone unit 400 can be supported by wheels 408 so that it canbe easily moved to a position within the room where it is most likely toencourage air flow circulation throughout the entire room. Also, thestand-alone unit 400 can be utilized as an exhaust unit from a room inwhich undesirable fumes are present which one does not wish to exhaustinto the atmosphere without treating them first. For instance, paintshops or in other industrial plants, the stand-alone unit 400 can bebuilt into a roof or any exterior wall or it can be adapted for windowsupport in the same fashion as a room air conditioner. In any event,contaminated air is cleansed before reaching the atmosphere.

The invention can be used to improve existing filter types. For example,the invention can be used together with electrostatic filters orphotocatalytic electrostatic filters. Such combinations can beintegrated into a single filter, or used together in a serialarrangement. Photocatalytic electrostatic filters are described in U.S.Pat. No. 5,993,738 to the same inventor as the current application andis hereby incorporated by reference in its entirety into the currentapplication.

A method for disinfecting a fluid stream containing microorganisms orchemical contaminants includes the steps of providing a fluid stream andcontacting the fluid stream with a plurality of photocatalyst surfaceswhich generate electrons and holes upon suitable irradiation. Thephotogenerated electrons and holes are directed away from one another tolimit their recombination. At least a portion of the microorganisms orchemical contaminants are then photocatalytically oxidized at a rateenhanced by the reduced recombination rate provided by the invention.

The invention can be used in products for cleaning indoor air inbuildings, and other enclosed spaces such as automobiles, airplanes andhospitals. The invention can also be useful against bioterrorism throughapplication in mail rooms, train stations' and airports.

The invention can be applied to envelopes and other surfaces, such asfloors and walls, upon which, a layer of photocatalyst particles may bepainted or otherwise applied and secured thereto. For example, appliedto envelopes, an envelope can include a thin layer of photocatalystparticles, such as nanometers to tens of nanometers thick, on itssurfaces. To provide disinfection, a metal grid can brought into contactwith the photocatalyst layer provided by the envelope. The metal gridcan be tied to an external ground, or between an external power supplyand an external ground. Disinfection is then initiated by exposure ofthe coated envelope to light of the appropriate wavelength, wherein asubstantial portion of the photogenerated electrons are directed awayfrom photocatalytically generated holes thus reducing the rate ofelectron-hole recombination reactions and accordingly increasing thespeed and efficacy of contaminant destruction. This method is preferablypracticed using automated equipment that passes the articles through alight emitting arrangement, such as at a mail facility.

When products based on this process are used in building ventilationsystems, they will help persons with asthma and allergy problems, andprevent the spread of disease through air. The invention can be used tohelp the military to combat chemical and biological warfare. Theinvention can also be used to trap spreading spores such as anthrax. Inaddition, the invention can be used to improve cleanliness in cleanrooms which is known to help improve yields for products fabricatedusing high technology manufacturing processes, such as semiconductors.

The invention can also be used for liquid purification, such as waterpurification. FIG. 5 shows a water purification system 500 based on anembodiment of the invention. System 500 includes substrate layer 510coated with a photocatalyst layer 515. A current collecting grid 520 isdisposed on the photocatalyst layer 515. A metal back plate 505 isdisposed on substrate 510 opposite photocatalyst layer 515. A connector540 electrically couples grid 520 and metal back plate 505. Light 530from a suitable light source (not shown) incident on photocatalyst layer515 generates hole-electron pairs. Electrons are collected by grid 520and are shunted to metal back plate 505 via connector 540, thusincreasing the rate of photoelectrochemical removal of contaminants inthe water. Although not shown, a bias can be applied between grid 520and metal back plate 505 to further increase the rate ofphotoelectrochemical removal of contaminants in the water.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples which follow are intendedto illustrate and not limit the scope of the invention. Other aspects,advantages and modifications within the scope of the invention will beapparent to those skilled in the art to which the invention pertains.

1-22. (canceled)
 23. A coating for disinfecting surfaces, comprising: aphotocatalyst layer comprising a plurality of photocatalyst particlesfor photogenerating electrons and holes, said photocatalyst particlessecured to a solid surface, and an electrically conductive layerdisposed on and in electrical contact with said photocatalyst layer forremoving a portion of said photogenerated electrons, said electricallyconductive layer being interposed between said source of photons andsaid photocatalyst layer wherein incident light from said source ofphotons is transmitted by said electrically conductive layer to said topsurface of said photocatalyst layer, said electrically conductive layerreducing an electron-hole recombination rate involving saidphotogenerated electrons and holes resulting in an increase in a removalrate of microorganisms or chemical contaminants from said surface. 24.The coating of claim 23, wherein said solid surface comprises at leastone selected from the group consisting of wall coverings, floorcoverings, envelopes, packages, and clothing articles.
 25. The coatingof claim 23, wherein said electrically conductive layer comprises anelectron conducting metallic grid in contact with said photocatalystlayer, said metallic grid including a plurality of metallic strips. 26.The coating of claim 23, wherein said photocatalyst layer includes aplurality of metal ions.
 27. The coating of claim 23, wherein saidelectrically conductive layer comprises silver.
 28. The coating of claim23, wherein said coating is disposed in an air supply register of aheating, ventilating and air conditioning system or in an air duct. 29.A method for disinfecting surfaces, comprising the steps of: providing asurface including a photocatalyst layer comprising a plurality ofphotocatalyst particles secured to said surface for photogeneratingelectrons and holes, said photocatalyst layer having a top and a bottomsurface, wherein said bottom surface is secured to said surface;contacting an electrically conducting layer to said photocatalyst layer,wherein said electrically conducting layer is disposed on and inelectrical contact said photocatalyst layer; illuminating saidphotocatalyst layer with a source of photons having a wavelengthcorresponding to at least a band gap energy of said photocatalyst,wherein incident light from said source of photons is transmitted bysaid electrically conductive layer to said top surface of saidphotocatalyst layer to produce photogenerated electrons and holes, andremoving a portion of said photogenerated electrons generated via saidelectrically conductive layer, wherein an electron-hole recombinationrate involving said photogenerated electrons and holes is reducedincreasing a removal rate of microorganisms or chemical contaminantsfrom said surface.
 30. The method of claim 29, wherein said providingstep comprises applying said photocatalyst layer to at least oneselected from the group consisting of wall coverings, floor coverings,envelopes, packages, and clothing articles.
 31. The method of claim 29,wherein said contacting and said illuminating steps are automated. 32.The method of claim 29, wherein said photocatalyst layer includes aplurality of metal ions.
 33. The method of claim 29, wherein saidelectrically conductive layer comprises a grid, said grid including aplurality of strips, spaces between said strips allowing said incidentlight to reach said photocatalyst layer.
 34. The method of claim 29,wherein said electrically conductive layer comprises a continuous sheet,said electrically conductive layer being substantially opticallytransparent for said wavelength of said photons to allow said incidentlight to be transmitted therethrough to reach said photocatalyst layer.